Typical displays include a cathode ray tube (CRT), a liquid crystal display (LCD) using electro-optic efficiency, a plasma display panel (PDP) using gas discharge, and an electro luminescence display (ELD) using electroluminescence. Among them, the research on the LCD has been actively performed.
Recently, the LCDs have been developed enough to serve as a flat display, so that the LCDs have been used for the monitor of a desktop computer, a TV, and a large-scaled information display, as well as the monitor of a laptop computer.
Since most of the LCDs are light receiving devices that adjust an amount of light received from an external device to display an image, an additional light source, that is, a back light unit to irradiate light to an LCD panel is required.